Using Pstn Reachability to Verify Voip Call Routing Information

1. A method comprising: initiating a public switched telephone network (PSTN) call to a destination telephone number; storing an attribute of the PSTN call with a processor; verifying with the processor that a Voice-over-Internet-Protocol (VoIP) call agent owns the destination telephone number by receiving an indication from the VoIP call agent that the VoIP call agent has knowledge of the attribute of the PSTN call initiated to the destination telephone number; and providing Instant Messaging (IM) with the processor over a VoIP call to the VoIP call agent based on the verification that the VoIP agent owns the destination telephone number.

2. The method of claim 1 wherein the verifying comprises transmitting a query to the VoIP call agent for proof of knowledge of the attribute of the PSTN call, and wherein the receiving the indication comprises receiving a response to the query from the VoIP call agent that demonstrates knowledge of the attribute of the PTSN call.

3. The method of claim 1 further comprising discovering the VoIP call agent by querying a peer-to-peer network for a match with the destination telephone number.

4. The method of claim 3 wherein one or more members of the peer-to-peer network comprises VoIP call agents including the VoIP call agent.

5. The method of claim 1 further comprising discovering the VoIP call agent by querying a central server for a match with the destination telephone number.

6. The method of claim 1 wherein the attribute of the PSTN call comprises a dual-tone multi-frequency (DTMF) value transmitted over the PSTN call.

7. The method of claim 1 wherein the verifying that the VoIP call agent owns the destination telephone number is performed before the PSTN call ends.

8. A system comprising: an originating Voice-over-Internet-Protocol (VoIP) call agent comprising a processor and a network interface, wherein the originating VoIP call agent is configured to: initiate a public switched telephone network (PSTN) call to a destination telephone number with the processor over a PSTN; store an attribute of the PSTN call; verify that a destination Voice-over-Internet-Protocol (VoIP) call agent owns the destination telephone number based on receipt of an indication from the destination VoIP call agent over the network interface that the destination VoIP call agent has knowledge of the attribute of the PSTN call initiated to the destination telephone number; and provide Instant Messaging (IM) between the originating VoIP call agent and the destination VoIP call agent over a VoIP call based on the verification that the destination VoIP agent owns the destination telephone number.

9. The system of claim 8 , wherein the originating VoIP call agent is further configured to provide the IM over the VoIP call before the PSTN call ends.

10. The system of claim 8 , wherein the originating VoIP call agent is further configured to provide the IM over the VoIP call when the destination VoIP agent is verified as owning the destination telephone and to provide the IM over the PSTN when the destination VoIP agent is not verified as owning the destination telephone number.

11. The system of claim 8 , wherein the originating VoIP call agent is further configured to provide a video portion of an audio-visual communication session over the VoIP call and to provide a voice portion of the audio-visual communication session over the PSTN call in response to the verification that the destination VoIP call agent owns the destination telephone number.

12. The system of claim 8 , wherein the attribute of the PSTN call comprises at least one of a start time of the PSTN call or an end time of the PSTN call.

13. The system of claim 8 further comprising a cache, wherein the originating VoIP call agent stores VoIP call routing information in the cache in response to the verification that the destination VoIP agent owns the destination telephone number, and provides the IM over the VoIP call in response to the VoIP call routing information being found in the cache.

14. The system of claim 8 wherein the receipt of the indication includes a use of an authentication protocol to verify that the destination VoIP call agent knows a password mapped from the attribute of the PSTN call without transmission of the attribute of the PSTN call between the destination VoIP call agent and the originating VoIP call agent.

15. Logic encoded in a tangible non-transitory computer readable storage medium executable with a processor to: verify that a Voice-over-Internet-Protocol (VoIP) call agent owns a destination telephone number based on receipt of an indication from the VoIP call agent that the VoIP call agent has knowledge of an attribute of a public switched telephone network (PSTN) call initiated to the destination telephone number over a public switched telephone network; store VoIP call routing information in a cache in response to the verification that the VoIP call agent owns the destination telephone number; and provide Instant Messaging (IM) over a VoIP call to the VoIP call agent in response to the VoIP call routing information being found in the cache.

16. The computer readable storage medium of claim 15 , wherein the logic encoded in the computer readable storage medium is further executable to: search the cache for the VoIP call routing information based on the destination telephone number; initiate the VoIP call if the VoIP call routing information is found in the cache; and provide the Instant Messaging (IM) over the PSTN if the VoIP routing information is not found in the cache.

17. The computer readable storage medium of claim 15 , wherein the PSTN call attribute comprises at least one of a caller identification, a call length, or a call signature.

18. The computer readable storage medium of claim 15 , wherein the PSTN call attribute comprises a call signature, wherein the call signature is based on at least one of frequency spectrum analysis, silence detection, or a password.

19. The computer readable storage medium of claim 15 , wherein the verification is performed after the PSTN call ends.

20. The computer readable storage medium of claim 15 , wherein the logic encoded in the computer readable storage medium is further executable to identify the VoIP call agent from a search of a registry for the destination telephone number prior to the verification.